Magnetron



Patented July 3, 1951 MAGNETRON Jerome Kurshan, Princeton, N. J., assignor to Radio Corporation of America, a corporation of Delaware Application November 22, 1946, Serial No. 711,683

Claims. 1

This invention relates to high frequency oscillators incorporating cavity resonators and more particularly to arrangements for tuning such oscillators in order to vary the frequency of the oscillations.

Various types of generators for ultra-high frequency oscillations are known in the art. Representative among them is the magnetron to which this invention particularly pertains. Essentially in one form the magnetron comprises an evacuable housing in which there is an anode in the form of a solid conducting block with a central circular aperture for accommodating an axial electron emitting cathode. Cooperating with this aperture and radially extending therefrom are a plurality of cavity resonators. These resonators may be formed by vanes dividing the circle around the cathode into a number of cavities which open into the central aperture and are closed at the other end by the block. The cavities also communicam with each other at the sides of the vanes. In this manner there exists a close coupling between the cavities inasmuch as the high frequency magnetic field interlinks the cavities at their open sides, whereas the high frequency electric field provides such interlinkage in the central aperture where there is an interaction space between the electric field and the electrons emitted from the cathode. In the operation of the magnetron there is also provided a constant magnetic field parallel to the cathode by means of a permanent magnet or an electro-magnet located outside of the housing.

The frequency of a magnetron is determined by the shape and size and particularly the radial length of the sector-shaped resonators. In view of the fact that these resonators are formed within a solid conducting block, their shape and size is predetermined and cannot be changed once the block is a part of a completed structure. This drawback of a fixed frequency in a magnetron requires costly tolerances to be met in manufacture. The replacement of magnetron tubes in the field is also difficult because the new tube has to be exactly identical with the old one.

It has been proposed to build a tunable magnetron in several ways. One solution advanced provides cup-shaped members which are inserted near the interaction space in the region of the high electric field whereby the effective capacity of the resonator may be varied. Attempts have been made also to make one of the walls of the resonator movable and in this manner change the physical length of one or more resonators. In making one of the walls movable, the entire distributed electric and magnetic fields of the resonator are changed together with its actual size. This can be accomplished for example by making the end Wall of the radially extending cavity of the resonator movable toward the center of the block. Needless to say, this scheme requires difiicult and complicated mechanical assembly, particularly when all the cavities are to be altered simultaneously. However, satisfactory tuning of the magnetron may also be effected by altering only one of the cavities. It was mentioned before that all the resonators are closely coupled. Hence, altering one resonator within certain limits affects the entire composite structure.

In the present invention cognizance is taken of this fact that one resonator may be tuned in order to produce a frequency change of the magnetron. However, instead of making the actual physical length variable, which is difiicult to achieve mechanically and impractical in mass production, it is proposed to restrict only the high frequency magnetic field of one resonator. This is accomplished by simple mechanical means without disturbing the electric field and leaving the actual physical length of the resonator intact.

Accordingly, it is an object of this invention to provide a magnetron with means whereby the frequency thereof may be adjusted over a useful range.

Another object of the invention is to provide a simple, efficient structure as an integral part of a magnetron for adjusting its natural frequency.

An advantage of the invention is that the effective inductance of a resonator of the magnetron is varied without changing its actual physical length and without disturbing its effective capacity.

Another advantage of the invention is that the magnetron in accordance therewith is easy to assemble and requires only average tolerances in manufacture.

Other objects and advantages will be apparent from the following description of the invention, pointed out in particularity in the appended claims and taken in connection with the accompanying drawing in which:

Fig. 1 is a side elevational view in cross-section of multi-cavity magnetron incorporating the tuning arrangement in accordance with the invention;

Fig. 2 is a top view and section along line 2-2 of Fig. 1;

Fig. 3 is a performance chart showing by means of a curve the frequency variation with respect to number of turns of the tuning shaft.

Referring to the drawing, the magnetron per se is illustrated chiefly to show the tuning arrangement. For this reason non-essential parts as far as the invention is concerned have been omitted.

In the evacuated housing 3 and mounted on the base header 4 is an anode block 5 which is divided into a number of resonators s in the form of selectors of a circle around the axial cathode i in the central aperture '8. Each resonator 6 consists of the side walls or vanes H] which form spaced spokes around the central aperture 8 as the hub. The resonator cavities open into the central aperture 8 as well as on both sides into the space between the block 5 and the housing 3.

One of the resonators 6 is provided with an opening Ii in the side of the block 5 through which an elongated conducting member comprising a rod l2 may enter the cavity of the resonator. A guide bushing if; is supported on the housing 3 to which it is solidly attached, for example by welding or soldering in order ot provide a vacuum-tight seal. The bushing l3 comprises a radial flange and an axially extending sleeve which guides the rod 2. The rod it. has a surface of high conductivity to minimize conduction losses. A flexible tubular metallic housing id is brazed to the flange of the bushing iii. The other end of the housing It is closed off by a bushing 15 to which it is also soildly attached in order to make a vacuum-tight joint. The housing is is of the type known as a Sylphon bellows and provides a vacuum-tight compartment with sufficient flexibility to permit the movement of the rod l2 in or out of the resonator is. The bushing 56 has a sleeve portion in which the outer end of the rod i2 is threaded and a cup-shaped portion into which is fitted a ball bearing ll fastened to the bushing 53 by means of screws l8. A threaded shaft is extends through the bearing 11 and is rotatably secured to it. A cover or yoke member 213 fitting over the afore-described assembly has a tapped opening which engages a threaded portion of the shaft I9 and is secured to the flange of the bushing 13 by any suitable means, such as the screws 22.

In order to provide an axially tight fit between the bushing i6 and the bearing H, and thereby eliminate baclnlash or lost motion when the shaft is is turned, a thrust bearing is provided comprising a spring plate 23 located in the cup of the bushing i6 and a call 2 2 rolling in a depression 25 at the head of the shaft it.

As the shaft is is turned, the housing M will either be compressed or expanded within the cover 29 depending upon the direction of rotation. To obtain an increase in frequency, the shaft is is turned in the direction which corn- .presses the housing H3 causing further penetration of the rod i2 into the cavity of the resonator 6. It is to be noted that the rod l2 has a tapered end which conforms to the angle between the sector walls is to permit penetration of the rod into the cavity of the resonator only up to a certain point. The degree of taper is so calculated that the end of the rod l2 should not go beyond the region of the magnetic field pattern WVhBIE the high frequency magnetic field strength is .relatively high and the high frequency electric field strength is low. Thus, from :a circuit standpoint, moving the rod l2 changes the effective inductance of the resonator. Greater penetration corresponds to lower inductance and a 4 higher resonant frequency. Insertion too far would result in reaching the predominantly electric field pattern of the resonator producing an opposing effect caused by the increase in effective capacitance as the rod enters the region of high electric field. The taper provides a simple means for preventing penetration beyond the magnetic field region which would cause a resultant change in the effective capacitance of the resonator. In addition, a stop is provided for the rod i2 also by the opposing faces of the sleeve portion of the bushing l6 and the sleeve portion of the bushing 43.

The simple tuning arrangement just described permits ease of manipulation and eliminatesthe necessity for varying the actual physical size of the resonator. Moreover, the tuning is restricted substantially to one component of the high frequency field of the resonator at a point where there is a higher ratio of physical change to corresponding electrical change. Consequently, larger increments of motion or" the mechanical assembly may be had for a certain frequency change than for similar variation of the effective capacitance.

By introducing the tuning rod through an opening in the side of the block, and perpendicularly to the original high frequency fields, a tuning arrangement inherently free from unwanted and troublesome resonances is obtained. There is little tendency for the high frequency fields, particularly the magnetic field, to couple to the structure supporting and enclosing the tuning rod. In a magnetron structure Where the tuning rod is introduced through the open sides of the resonator, that is, in the direction parallel with the original high frequency magnetic field there is a tendency for this field to couple with the control mechanism and supporting apparatus.

In a practical embodiment operating at 4:000 megacycles a frequency shift of 80 megacycles was produced with approximately 0.73 inch pen etration of the tuning rod. The curve in Fig. 3 illustrates the relation of frequency shift to the number of turns of the tuning shaft. It is seen that approximately '7 complete turns will give an 80 megacycle change in the output frequency of the magnetron.

While the invention herein described is applied to a magnetron for the purpose of illustration, the principles thereof have wider application. It can be applied to other tubes which have vacuum cavity resonators, for example, to velocity modulation type tubes. The shape and location of the tuning rod may be modified in a a manner which will be evident to those skilled in the art.

I claim:

l. A vane-type magnetron comprising ,an an ode block having a cylindrical aperture containing a plurality of spaced conducting vanes .eX- tending radially from a central interaction space and terminating at the wall of said aperture and forming cavity resonators each having the shape of a sector of a circle, a rod of conducting material extending outwardly from the side of said block and penetrable into the predominantly magnetic field region adjacent the outer end of one of said resonators, adjustable means supporting said rod for movement into and out of said resonator, and means preventing movement said rod into the predominantly electric field region of said resonator adjacent said-pa fi comprising a apered en po o of said rod similar in longitudinal sect n to said sec-ter 76 shaped resonators, whereby the effective inductonce of said resonator may be varied in accordance with the extent of penetration of said rod.

2. An electron discharge device of the magnetron type including an evacuated envelope, an anode block therein having a plurality of radially extending cavity resonators communicating with a central interaction space and terminating within said block, tuning means for said device comprising a conducting rod extending outwardly from the side of said block and through an aperture in the outer end of one of said resonators, said envelope having guide means for said rod adjacent said aperture, a flexible elongated tubular housing surrounding the outwardly extending portion of said rod and hermetically sealed to said guide means, said rod being secured to said housing remote from said guide means, actuating means for said rod comprising a. cover secured to said guide means and surrounding said housing, first screw means attached to said housing and extending through said cover and second screw means on said cover engaging said first screw means whereby upon relative actuation of said screw means in one direction said housing may be compressed causing movement of said rod into one of said resonators and upon relative actuation of said screw means in another direction said housing may be expanded causing movement of said rod out of said resonator.

3. An electron discharge device in accordance with claim 2 wherein said first screw means is rotatably secured in a bearing attached to said housing, said bearing having a spring pressing said first screw means tightly against said bearing, whereby lost motion in said actuating means in the direction of movement is prevented.

4. An electron discharge device in accordance with claim 2, wherein said outwardly extending portion of said rod is provided with a shoulder engageable with a cooperating shoulder on saidguide means when said rod is moved into said resonator to limit the penetration of said rod to the outer portion only of said resonator.

5. An electron discharge device for ultra high frequencies including a cavity resonator in which an alternating electric field and an alternating magnetic field are developed in different regions thereof during operation, said magnetic field region determining the efiective inductance oi? said resonator, means for controlling the frequency of said oscillations comprising an elongated conducting member penetrating said resonator in the region of said magnetic field, for changing said effective inductance, and adjustable means supporting and guiding said conducting member comprising elongated tubular guide means rigidly mounted adjacent said cavity resonator, said conducting member extending through said guide means, a cover rigidly mounted on said guide means and surrounding said conducting member, first screw means attached to said conducting member and extending through said cover, and second screw means on said cover engaging said first screw means,

whereby upon relative rotation of said screw means said conducting member may be moved into or out of said resonator, said conducting member being provided with a shoulder engageable with a cooperating shoulder on said guide means when said rod is moved into said resonator to limit the penetration of said conducting member to the magnetic field region only of said resonator.

JEROME KURSHAN.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 2,342,897 Goldstine Feb. 29, 1944 2,415,253 Linder Feb. 4, 1947 2,418,469 Hagstrum Apr. 8, 1947 2,419,572 Laico et al. Apr. 29, 1947 2,445,282 Slater July 13, 1948 2,466,765 Hartman Apr. 12, 1949 

